1. Field of the Invention
This invention relates generally to a laser-plasma, extreme ultraviolet light source and, more particularly, to a laser-plasma, extreme ultraviolet light source that provides synchronized laser pulses and a target droplet delivery rate so that buffer droplets are provided between consecutive target droplets.
2. Discussion of the Related Art
Microelectronic integrated circuits are typically patterned on a substrate by a photolithography process, well known to those skilled in the art, where the circuit elements are defined by a light beam propagating through or reflected from a mask. As the state of the art of the photolithography process and integrated circuit architecture becomes more developed, the circuit elements become smaller and more closely spaced together. As the circuit elements become smaller, it is necessary to employ photolithography light sources that generate light beams having shorter wavelengths and higher frequencies. In other words, the resolution of the photolithography process increases as the wavelength of the light source decreases to allow smaller integrated circuit elements to be defined. The current state of the art for photolithography light sources generate light in the extreme ultraviolet (EUV) or soft x-ray wavelengths (13-14 nm).
U.S. patent application Ser. No. 09/644,589, filed Aug. 23, 2000, entitled “Liquid Sprays as a Target for a Laser-Plasma Extreme Ultraviolet Light Source,” and assigned to the assignee of this application, discloses a laser-plasma, EUV radiation source for a photolithography system that employs a liquid as the target material, typically xenon, for generating the laser plasma. A xenon target material provides the desirable EUV wavelengths, and the resulting evaporated xenon gas is chemically inert and is easily pumped out by the source vacuum system. Other liquids and gases, such as krypton and argon, and combinations of liquids and gases, are also available for the laser target material to generate EUV radiation.
The EUV radiation source employs a source nozzle that generates a stream of target droplets in a vacuum environment. The droplet stream is created by allowing a liquid target material (typically xenon) to flow through an orifice (50-100 microns diameter), and perturbing the flow by voltage pulses from an excitation source, such as a piezoelectric transducer, attached to a nozzle delivery tube. Typically, the droplets are produced at a rate defined by the Rayleigh instability break-up frequency (10-100 kHz) of a continuous flow stream. The droplets are emitted from the nozzle where they evaporate and freeze. The size of the orifice is set so that as the droplets freeze and are reduced in size, they are of a size at the ionization region where ionization by a high intensity laser pulse will generate significant EUV radiation, without allowing pieces of frozen xenon to escape ionization, and possibly damage sensitive optical components.
To meet the EUV power and dose control requirements for next generation commercial semiconductors manufactured using EUV photolithography, the laser beam source must be pulsed at a high rate, typically 5-20 kHz. It, therefore, becomes necessary to supply high-density droplet targets having a quick recovery of the droplet stream between laser pulses, such that all laser pulses interact with target droplets under optimum conditions. This requires a droplet generator which produces droplets within 100 microseconds of each laser pulse.
When the laser source is operated at these frequencies for a liquid droplet stream generated at the Rayleigh frequency for an orifice of the desirable size, closely spaced droplets are generated, where the spacing between droplets is approximately nine times the droplet radius. Due to this proximity, a target droplet currently being ionized adversely affects successive droplets in the stream. Thus, the successive droplets are damaged or destroyed prior to being ionized by the laser beam.
One approach for preventing successive target droplets from being effected by ionization of a preceding target droplet would be to have the laser pulse hit each droplet immediately as it emerges from the nozzle orifice. However, this would result in plasma formation very close to the nozzle orifice, providing an excessive heat load and causing plasma-induced erosion of the nozzle orifice.
Another approach would be to energize the piezoelectric transducer at frequencies other than the natural Rayleigh break-up frequency of the target material. In other words, the frequency of the droplet formation can be adjusted away from the Rayleigh frequency, and the droplet spacing can be varied. This will allow some adjustment of the droplet frequency to match the laser pulse frequency. However, operating the transducer at a frequency other than the Rayleigh break-up frequency adversely affects the ability to create a consistent stream of droplets. Because xenon is a gas at room temperature and pressure, the xenon gas is cooled to, for example, −100° C., to liquify it. Drop on demand generators are difficult to control to provide droplets of the right size at the right time because of the surface tension properties of liquid xenon.
Another approach would be to increase the size of the nozzle orifice so that the droplets are generated at the Rayleigh break-up frequency less often. However, this leads to droplets of too large a size for the laser ionization process, possibly causing component damage resulting from unionized frozen xenon.